Thomas L. Brunell and William Koetzle, "A
Divided-Government-Based Explanation for the Decline in
Resignations from the US Senate, 1834-1996," Party
Politics, 5 (October 1999), 497-505.

First Paragraph:

The US Senate experienced the resignation of two of
its most senior members during the 104th Congress.
Senators Robert Dole (Republican, Kansas) and Robert
Packwood (Republican, Oregon) both left office before the
expiration of their terms, Dole to run for president and
Packwood to run for cover. These highly publicized
resignations raise an interesting question in modern
American politics: why are fewer and fewer senators
resigning their seats before the end of their term? In
the 19th century, for example, it was quite common for
senators to resign from office and not serve out their
term. Between 1834 and 1850 no less than 47 senators
resigned from office, contrasted with only 13 Senate
resignation in the period 1980-96. There are undoubtedly
numerous reasons for this decline. Two related causal
factors are the increase in careerism in Congress and the
concomitant institutionalization of the body (Polsby,
1968; Rohde et al., 1985).

Figures and Tables:
Table 1: Breakdown of US Senate resignations, 1834-1996
Figure 1: Total Senate resignations and propostion of
senators who could resign in a way favorable to their
party

Last Paragraph:
Our starting point was the puzzling decline in the number of
resignations from the US Senate. While the rise of
institutionalization of Congress is undoubtedly linked to
this phenomenon, we have shown that this decline is also
linked to a change in the opportunity structure.
Specifically, the data presented here demonstrate that
senators are significantly more likely to resign when their
party will continue to control that seat. Given the natural
position of senators in their party -- they are generally
high-ranking and well-respected members of their respective
political parties -- this is not surprising. The incentive
for senators to resign only when their party will retain
control of that seat has remained constant over time. Yet
the increased incidence of divided party control between
senators and the body with the power to appoint a
replacement (i.e. the state legislature or the governor) has
resulted in a marked decrease in the number of senators who
are able to resign with the assurance that a replacement
will be from the same party. Occasionally resignations from
the Senate still do occur and when they do, they remain
overwhelmingly of this type, despite the fact that it is now
more difficult to resign with the guarantee that the same
party will retain control of the seat.